The field of the invention is the non-invasive determination of tissue elasticity with medical imaging techniques, such as ultrasound or magnetic resonance elastography imaging.
The physician has many diagnostic tools at his or her disposal which enable detection and localization of diseased tissues. These include x-ray systems that measure and produce images indicative of the x-ray attenuation of the tissues and ultrasound systems that detect and produce images indicative of tissue echogenicity and the boundaries between structures of differing acoustic properties. Nuclear medicine produces images indicative of those tissues which absorb tracers injected into the patient, as do PET scanners and SPECT scanners. And finally, magnetic resonance imaging (“MRI”) systems produce images indicative of the magnetic properties of tissues. It is fortuitous that many diseased tissues are detected by the physical properties measured by these imaging modalities, but it should not be surprising that many diseases go undetected.
Elastography is a technique for quantitatively assessing the mechanical properties (e.g., stiffness) of soft tissues. In the most common embodiment, it includes measuring the displacement of tissue in response to an applied force or stress, and using that information to determine the stiffness of the material. The applied stress can be static, quasi-static, transient, or dynamic (with the most common dynamic case being harmonic motion). Techniques for using MRI or ultrasound to perform elastography are well known.
In most vessels in the body, blood pressure can be measured either externally or internally via a catheter. However, the portal vein, the principal blood supply to the liver, lies deep within the body and has capillary beds on both sides that make it difficult to reach by catheter. Portal venous hypertension (PVH) is one of the most important consequences of chronic liver disease, leading to gastroesophageal varices and potential life-threatening hemorrhagic complications.
Currently, portal venous pressure is assessed by hepatic vein catheterization and measurement of hepatic venous “wedge pressure”. Unfortunately, the highly invasive nature of this procedure severely limits the potential to use portal pressure data to guide clinical management. Conventional MRI and CT can only diagnose PVH by detecting late consequences of portal vein hypertension such as varices, splenomegaly and ascites. Thus, although portal pressure is considered an important indicator of various disease processes, it is currently very difficult to measure.
Previous studies with both ultrasound and MR elastography have established that there is a strong relationship between increasing liver stiffness and the severity of hepatic fibrosis. It is also know that portal venous pressure increases systematically with the severity of liver fibrosis.